In order to implement a stable image density constantly in electrophotographic image forming apparatuses, a toner patch (or, gradation pattern) for detecting the image density is formed conventionally on an image bearing member such as a photoreceptor, for example. The density of the gradation pattern is detected with an optical detecting unit and the potential applied for image development is suitably adjusted according to the results obtained from the detection, which is carried out specifically by changing an LD (laser diode) power, a charging bias, and a developing bias.
As the optical detecting unit for detecting the gradation patterns, a reflection type optical sensor is conventionally known including a light emitting diode (LED) as light source means, and a photodiode (PD) or a phototransistor (PTr) as photoreceptor means.
As the configuration of the optical sensor, there are three types; (A) a first type of the sensor configured to detect only regular reflection light, as illustrated in FIG. 2 (See for example, Japanese Laid-Open Patent Application No. 2001-324840), (B) a second type to detect only diffuse reflection light, as illustrated in FIG. 3 (Japanese Laid-Open Patent Application No. H5-249787 and Japanese Patent Publication No. 3155555), and (C) a third type to detect both regular and reflection light, as illustrated in FIG. 4 (Japanese Laid-Open Patent Application No. 2001-194843).
Reference numerals in FIGS. 2 through 5 are 50A, 50B, and 50C for denoting element holders, 51 for an LED, 52 for a regular reflection photodetector, 53 for a target surface to be detected, 54 a toner gradation pattern on the target surface, and 55 for a diffuse reflection photodetector, respectively.
A fourth type of the sensor (D) illustrated in FIG. 5 has also been used recently, in which a beam splitter is provided on the optical path on both sides of light emission and reception (Japanese Patent Publication No. 2729976 and Japanese Laid-Open Patent Applications No. H10-221902 and 2002-72612).
Reference numerals in FIG. 5 are 56 for denoting an LED, 57 and 58 for beam splitters, 59 for a first photodiode as a light receiving unit for P-wave light (regular reflection light), and 60 for a second photodiode as another light receiving unit for S-wave light (diffuse reflection light), respectively.
As illustrated in the abovementioned disclosures which describe primarily on the formation of color images, a change in the image density leads to a change in hue in the color image forming apparatuses. Therefore, it is important to accurately detect the amount of toner adhered on the gradation patches or patterns in use for detecting density control, in order to stabilize the image density, and properly implement the control according to the results obtained from the detection.
By the above term, “image density” to be stabilized, it is meant the “image density of output image”. Conventional monochrome image forming apparatuses perform the detection on the density of photosensitive materials.
By contrast, it is preferable in the color image forming apparatus to perform density detection on the transfer belt immediately before the transfer onto a paper sheet. In addition, since one of the purposes of the present image density control is to implement the control such that the maximum amount of adhesion is brought to a target value, it is desirable for accurate detection be feasible up to the range of high amount of the adhesion.
However, it has been difficult in conventional detection methods to detect the amount of adhesion accurately and constantly over the entire range of the amount of the adhesion.